1. Field of the Invention
This invention relates to electrical contactors, and more particularly to the control of, and communication links with microprocessor based contactors, and especially in multiple contactor applications.
2. Background Information
Contactors are switches which connect medium to large power consuming electrical loads, such as for instance electric motors, to a commercial power system. When combined with overload protection for a motor, the device is referred to as a motor starter. As used herein, the term "contactor" is intended to denote throughout, power switches used either with or without overload protection.
In a number of applications, multiple contactors are operated in a coordinated manner to perform a desired task. For instance, forward and reverse contactors are used to control the direction of rotation of a three-phase motor by selectively providing opposite phase rotations and in another application, two pairs of contactors are used to alternately connect a three-phase motor in a Y configuration for start-up and low speed operation, and to switch to a delta configuration for high speed operation as the motor speeds up.
Currently, push button stations are used to control such multiple contactor systems as well as for controlling single contactors remotely. These push button stations require numerous leads between the stations and the contactors, especially for providing interlocks in the multiple contactor configurations. Furthermore, they use 120 or line voltage signals requiring appropriate insulation.
Traditionally, contactors have been electromechanical devices. Recently, microprocessor based contactors have been developed, such as the contactors described in U.S. Pat. No. 4,893,102. The programmability of the microprocessor based contactor provides the capability for enhanced performance and versatility of the contactor. However, even the microprocessor based contactors are still controlled remotely, both singly and in multiple contactor installations, through push button stations which, as discussed above, require multiple connections at 120 or line voltage. Some contactor installations have low voltage communication links, but such links to date transmit only status information and other data, and do not provide remote control for operation of the contactors, except in some instances to trip the contactor. They do not provide a capability of closing the contactor, such as for starting a motor, although in some instances they might enable an automatic restart capability within the contactor.
U.S. Pat. No. 4,819,118 discloses a reversing motor control system in which the active one of the forward and reverse contactors, referred to as the "talker", continuously supplies a calculated thermal profile to the nonactive, "listener", contactor, so that upon reversal of the motor direction the former "listener" contactor becomes active with the current thermal profile. However, this system still utilizes a conventional push button station to control contactor operation.